This application claims priority from an earlier filed German Patent Application DE 198 58 428.8-52 filed on Dec. 17, 1998, which German application is incorporated herein by reference.
The present invention relates generally to the field of analytical instruments used for determining a location of an object, such as a substrate in an analytical instrument. More particularly, the invention relates to a displaceable X/Y coordinate measurement table used for interferometrically determining the coordinates of the substrate. The X/Y coordinate measurement table of the present invention is designed in such a way that the substrate positioned on the measurement table is accessible from above and from below.
Measurement tables are used in wafer steppers and in high-precision coordinate measuring instruments, such as interferometers. The measurement mirrors mounted on the tables make it possible to determine interferometrically the current position of the table while the table itself supports the analyzed or processed substrates.
A measurement table of that type is described in German Patent Application DE 198 19 492.7-52 as a component of a coordinate measuring instrument used for a highly accurate coordinate determination of structures on substrates, e.g. masks and wafers, but, in particular, of structures on transparent substrates. The measurement table described in that Application is vertically and horizontally displaceable and has a receiving rim for receiving the substrate. In addition, the measurement table is equipped with a frame-shaped opening, so that the received substrate is accessible from both above and below. Flat mirrors attached to two mutually perpendicular sides of the measurement table serve as measurement mirrors for a laser interferometer system that determines the X/Y position of the measurement table.
The described measuring instrument comprises a reflected-light illumination device and a transmitted-light illumination device having a common optical axis through an opening in the measurement table. Also present are an imaging device and a detector device for observing the analyzed structures. The measured coordinates of a particular analyzed structure are derived from the current interferometrically measured coordinates of the measurement table and from the coordinates of the analyzed structure relative to the optical axis of the table.
The article xe2x80x9cMaskenmetrologic mit der LEICA LMS IPRO fxc3xcr die Halbleiterproduktionxe2x80x9d [Mask metrology using the LEICA LMS IPRO for semiconductor production] by K.-D. Rxc3x6th and K. Rinn, Mitteilungen fxc3xcr Wissenschaft und Technik Vol. XI, No. 5, pp 130-135, Oct. 1997, describes a coordinate measuring instrument in which a measurement table of the aforesaid type is used to receive and analyze various substrates. The instrument comprises an interferometer and a separate interferometer measurement beam path for each coordinate axis X and Y of the measurement table. Two measurement mirrors located at the ends of the two interferometer measurement beam paths are attached to two mutually perpendicular sides of the measurement table. Such an instrument utilizes the two mirrors to interferometrically determine the coordinates of the measurement table, which coordinates are then used to calculate the coordinates of various structures on the substrates.
However, the above-described measuring instrument does not allow a user to perform high accuracy coordinate measurements of structures and defects at a nanometer scale, because in order to achieve such an accuracy, a number of instrumental error sources has to be taken into account and reduced or eliminated. For example, it would be desirable to eliminate such sources of measurement errors as thermal expansion of the materials of which various components of the instrument are made, changes in ambient temperature, humidity and air pressure affecting the wavelength of light in the interferometer, vibrations of the building where the instruments is located. The need, therefore, exists to provide a measuring instrument in which instrumental errors affecting the precision of the measurements are either minimized or eliminated.
The geometry of the measurement mirrors often also becomes a source of errors, since unidentified changes in the mirror geometry result in errors of the coordinates of the measurement table. This problem occurs generally in measurement tables which employ interferometric position determination, regardless of whether the measurement tables are utilized in coordinate measuring instruments or in steppers. The need, therefore, exists to provide an analytical instrument which is able to compensate for geometrical and other imperfections of the instrument.
The present invention addresses the above-described needs by providing an instrument in which a wide variety of instrument error sources is taken into account instrumentally and by providing various methods for minimizing or eliminating such error sources.
It is, therefore, an object of the present invention to minimize the effect of mechanical, physical, geometrical and other instrumental errors on the accuracy of high precision measurements performed by the instrument. For example, in order to minimize the effect of external building vibrations, the measuring instrument is mounted on a vibration-damped, air-mounted granite block with the air-mounted measurement table coupled to the block. In order to minimize the effects of changes in relative humidity and temperature of the surrounding environment, the measuring instrument of the present invention is housed in a climate-controlled chamber. Because the wavelength of the interferometer measurement light depends on air pressure, temperature, humidity, and the composition of air, the instantaneous value of the wavelength is continuously measured and monitored by taking into account a comparison measurement of a standard test article in calculating the final wavelength measurement result.
It is another object of the present invention to provide a device and method for error-free interferometric determination of the position of the measurement table by using software corrections of the instrumental errors. To achieve an error-free measurement, it is always desirable for the measurement mirrors to be flat, not tilted, and disposed orthogonally to one another. Since these criteria are hard to achieve instrumentally on a nanometer scale, the residual instrumental errors attributed to geometrical imperfections of the measurement mirrors, such as flatness, orthogonality, and tilting are detected and then compensated for by software corrections. In order to compensate for instrumental errors by using software corrections, the once-identified errors attributed to the mirrors should remain the same during subsequent measurements, i.e. even after the analyzed mask has been changed. This is achieved in the present invention by fabricating the measurement mirrors from a material with an extremely low coefficient of thermal expansion in order to minimize the effect of temperature changes on mirror geometry.
However, since the measurement mirrors are attached to the measurement table, the design of the measurement table itself becomes a critical component in measurement error reduction. For example, the flexible air-bearing system causes the measurement table to deform slightly, therefore, adversely affecting the mirror geometry. In addition, the analyzed masks and wafers have different dimensions and weights: the lightest mask, for example, weighs only about 80 g, but the heaviest weighs approximately 1.4 kg, while wafer chunks are even much heavier. If substrates with very different weights are placed on the measurement table one after another, the measurement table deforms differently each time.
The measurement table is also slightly deformed during displacement. The reason is that as the measurement table is displaced, it slides on the air bearings on the granite block the surface of which is not exactly flat on a nanometer scale. The measurement table is deformed during the displacement as a result of this residual irregularity of the granite block. These deformations of the measurement table in turn cause the measurement mirrors attached to it to deform and change their relative positions with respect to each other. In addition, due to a deformation of the measurement table, the position of the substrate relative to the measurement mirrors changes and the substrates become also deformed. While all these deformations and positional changes are very small, they can unpredictably affect the mirror geometry. In coordinate measuring instruments, such deformations and changes lead to errors in the measurement results. In the case of steppers, they result in positioning errors during the individual exposure operations.
It is therefore also an object of the present invention to provide an analytical instrument having an C/Y coordinate measurement table with measurement mirrors for interferometric position determination, in which the changes in the measurement table geometry caused by temperature and by load changes do not affect the geometry of the measurement mirrors. Moreover, in such an instrument the position of the measurement mirrors remains stable relative to the analyzed substrates. The measurement table of the present invention is suitable for analytical instruments using reflected or transmitted light for analyzing substrates of various weights, such as, for example, coordinate measuring instruments or wafer steppers.
The analytical instrument of the present invention comprises a displacable X/Y coordinate measurement table having a receptacle for an analyzed substrate and a mirror body carrying two measurement mirrors located at the ends of two separate interferometer measurement beam paths for determining the X and Y coordinates. The mirror body carrying the measurement mirrors and the receptacle for the substrate are implemented as separate components. In addition, the measurement mirrors define a surface region with a number of support points comprising spaced apart support points disposed substantially vertically on the upper and lower sides of the mirror body. Therefore, the support points disposed on the upper surface of the mirror body comprise the upper support points, the support points disposed on the lower surface comprise the lower support points. In the arrangement of the present invention the mirror body is supported on the measurement table only by the lower support points. The receptacle for the substrate or a substrate of suitable size is placed onto the upper support points.
The advantage of the arrangement is in the fact that the measurement table, the mirror body, and the receptacle for the substrate do not contact each other directly. When a substrate is placed on the measurement table, the support points receive the substrate and support it on the measurement table without passing the load on the mirror body. Each pair of oppositely spaced support points defines a vertical axis through which the applied weight is transferred onto the measurement table. As a result, the position of the mirror body remains undisturbed by the placement of the substrate or a receptacle for the substrate, so that the geometry of the measurement mirrors remains unaffected by varying weights of different substrates.
Deformations of the measurement table caused by temperature fluctuations or during displacement of the measurement table also have no effect on the mirror bodies, and thus on the measurement mirrors or their geometry. Since the substrate or the receptacle for the substrate placed onto the upper support points contacts the mirror body only at the upper support points and not over the entire surface, temperature differences between the substrate and the mirror body also have no effect on the measurement mirrors.
One of the embodiments of the present invention comprises a measurement table, a single piece mirror body fabricated from a material having a very low coefficient of thermal expansion and the two measurement mirrors integrated into the outer sides of the mirror body. The thermal expansion of the material, if any, is such that it does not affect the measurements performed with the help of the analytical instrument of the present invention. As an example, a coefficient of thermal expansion of the material can be from about 0.01xc3x9710xe2x88x926xc2x0C. to about 0.03xc3x9710xe2x88x926xc2x0C. The measurement mirrors can be integrated into the mirror body in various ways. One possibility is to manufacture the measurement mirrors separately as thin mirror surface elements. Each thin mirror surface element comprises a thin base surface onto which an actual measurement mirror is deposited. The mirror surface elements with the mirrors are then attached to the outer sides of the mirror body. For example, such an attachment can be accomplished by adhesive bonding. The problem that exists with every type of attachment, however, is that the mirror surface elements could become slightly distorted or bent. In order to prevent thermal or mechanical distortions, it proves advantageous to manufacture the base surface of the mirror surface elements and the mirror body of the same material.
The best way of integrating the measurement mirrors onto the mirror body is to mount the measurement mirrors directly onto the outer sides of the mirror body. As compared to the aforesaid methods, such a way is advantageous in that the measurement mirrors are not bent or distorted by a subsequent attachment procedure.
If the surface of the mirror body is sufficiently smooth, the measurement mirror can be formed by depositing it as one deposition layer onto the outer side of the mirror body. If, on the other hand, the surface of the mirror body contains irregularities such as pores, fine scratches or the like, then the measurement mirror is formed by depositing multiple layers onto the mirror body. In the case of multiple layer deposition, a precoat for the actual mirror is first deposited onto the mirror body and flattened by lapping or another high-quality surface preparation method. After lapping, the precoat becomes sufficiently flat and ready for deposition of a subsequent layer or layers. After the deposition is completed, the actual measurement mirror is mounted to the mirror body.
Regardless the number of layers forming the measurement mirror, the base coat for the subsequently deposited layers should be as flat as possible. When a lower layer deposited on the mirror body is subjected to lapping, the best flatness is achieved for a layer with a large surface. A large surface of the layer is usually optimally flat in the center and curves very slightly toward the edges. Therefore, the outer surfaces of the mirror body are designed to be sufficiently high, allowing the optimization of the surface processing to produce a wide flat surface for the measurement mirror. However, if the high outer edges make the mirror body too thick, it becomes too heavy. Manufacturing costs of a thick mirror body made of a material with an extremely low thermal expansion coefficient can become quite high, since such materials are usually very expensive.
It is, therefore, yet another object of the present invention to provide a particularly advantageous embodiment comprising the mirror body with an elevated rim having a particularly large, flat outer surface to which a measurement mirror is attached. The elevated rim is located on the surface of the mirror body and, in particular, on the three sides of the mirror body, forming a recessed area on the upper side of the mirror body. The recessed area is open at the fourth side, the fourth side being used for placing a substrate onto the upper support points. The advantage of this embodiment is that large cuter sides with sufficiently flat mirror surfaces can be manufactured of a minimal amount of material.
One of the embodiments of the present invention provides for a plurality of studs, wherein each stud has an upper and lower end and passes through a through opening in the mirror body and is secured in place in the opening. The lower ends of the studs support the mirror body on the measurement table, the upper ends of the studs support a substrate receptacle or a substrate itself, so that the receptacle or the substrate are supported in the instrument without contacting or deforming the mirror body. The preferably cylindrical studs are made of a material having an extremely low coefficient of thermal expansion, or they can be sintered in directly during manufacture of the mirror body. Other shapes of the studs can be used, depending on the shape of the through openings. The studs are inserted into the respective through openings of the mirror body stress-free and secured in place by fitting, adhesive bonding or any other suitable technique. Sintering has the advantage over adhesive bonding in that the sintered studs are reliably inelastic. When adhesive bonding is used to secure the studs in the openings, an adhesive that dries to become very firm is usually selected to result in firm, inelastic bonding between the mirror body and the studs.
In one of the embodiments of the present invention each stud is implemented with a support portion on which the mirror body is supported. The support portions of the studs secure the mirror body in its intended position and prevent the mirror body from sliding along the studs. If the studs are sintered into the mirror body during manufacturing, the support portions are built into the mirror body in order to maximize the area of contacting surfaces during the sintering.
The ends of the studs are preferably spherical to ensure the best possible support function. Since such spherical ends can wear faster than other pieces of the analytical instrument, the studs or even the mirror body itself would need to be replaced after a certain time. The replacement procedure can be fairly complex and expensive. Therefore, to eliminate such a problem the lower ends of the studs comprise recessions which receive ball shaped members attached to the recessions by adhesive bonding or other suitable means. Such ball shaped members can be easily and economically replaced after they are worn out without having to replace the studs or the mirror body. To ensure reliable positioning of the mirror body on the measurement table, the surface of the measurement table comprises a plurality of recessed areas suitable to receive the lower ends of the studs. These recessed areas are shaped and situated in such a way that they don""t affect the mirror geometry by displacing or deforming the mirror body.
In yet another embodiment of the present invention the plurality of recessed areas in the surface of the measurement table is provided to ensure stress-free thermal expansion of the mirror body when the lower ends of the studs are positioned in the recessed areas. In that embodiment a recessed area receiving the spherical lower end of the first stud or a ball shaped member is recessed into the rim region on the surface of the measurement table, therefore, securing the position of the stud in the recessed area.
In another particular embodiment of the present invention comprising three studs, the lower end of the second stud rests in a V-shaped groove disposed in the surface of the measurement table and extending parallel to an outer edge of the measurement table. The V-shaped groove has a longitudinal axis in line with the ball recess. The spherical lower end of the second stud engages into this V-groove and can be guided in a stress-free fashion in the groove if the mirror body thermally expands. The spherical lower end of the third stud moves unrestrictedly along a smooth sliding surface without distorting the mirror surface. Provided that the first stud is secured in its respective recessed area, the position and orientation of the mirror body relative to the measurement table remains unchanged, since in such an embodiment thermal expansion does not affect the outer edges of the mirror body and the measurement table, which outer edges remain parallel. The recessed area, the V-groove, and the flat sliding surface thus constitute a stress-free positive guidance system for the three-point mounting system of the mirror body unaffected by possible displacements of the mirror body relative to the measurement table caused by to thermal expansion.
Similarly, the above-described stress-free positive guidance system can be implemented in the second ends of the studs secured in the mirror body. In this embodiment, three recesses suitable for receiving and attaching three ball shaped members are provided in the surface of the measurement table. The lower ends of the three studs secured in the mirror body comprise a recess, a V-groove, and a flat sliding surface, respectively. The recess, the V-groove and the flat sliding surface rest on the upper side of the ball shaped members, allowing stress-free, positively guided thermal expansion or contraction of the mirror body and the measurement table relative to one another.
In yet another embodiment of the present invention the mirror body does not comprise the studs. Instead, the three studs are secured in the measurement table or shaped onto its surface. The three studs protrude and are removably inserted into three openings inside the mirror body. The upper end of the first stud comprises a recess, the upper end of the second stud comprises a V-groove in the direction of the recess, the upper end of the third stud has a flat sliding surface used to support the mirror body in a stress-free fashion. The recess, the V-groove, and sliding surface serve to receive the lower support points of the mirror body and to provide stress-free mounting of the mirror body.
The lower support points on the internal surface of the mirror body can be implemented in various configurations. In one example, the openings in the mirror body extend very deeply inside the mirror body, almost up to the upper surface of the mirror body. Fitted into the mirror body above the openings are balls with an upper surface extending above the upper surface of the mirror body and a lower surface extending into the openings. The lower surfaces of the balls contact the recess, the V-groove, and the flat sliding surface of the three studs providing the lower support points for the mirror body. As a result, the studs support the mirror body at these lower support points, so that the mirror body does not contact the measurement table. Thus neither thermal fluctuations nor mechanical distortions of the measurement table affect the mirror body or the mirror geometry. At the same time the upper surfaces of the balls extending above the surface of the mirror body provide the upper contact points for receiving an analyzed substrate or a receptacle for a substrate.
It is also contemplated in the present invention that instead of the balls, short semispherical studs can be utilized on the upper surface of the mirror body. In such an embodiment the semispherical upper ends of the studs extend above the upper surface of the mirror body and provide the upper support points. The lower ends of the semi spherically shaped studs extend into the openings in the mirror body and provide the lower contact points for supporting the mirror body on the measurement table. The lower support points rest stress free on the recess, the V-groove, and the sliding surface of the three studs inserted in the openings.
In yet another embodiment of the present invention the openings do not extend so deeply into the mirror body, leaving a thicker layer of the material above the openings, which results in greater warping stiffness of the mirror body as a whole.
To provide a stress-free positive guidance system for the lower support points, the recessed areas for accommodating three ball shaped members are made in the upper ends of the three openings. These three ball shaped members rest on the three studs containing a recessed area, a V-groove, and a sliding surface, respectively, and form the lower support points facing the studs. Alternatively, the stress-free positive guidance system can also be provided by a recessed area, a groove, and a sliding surface at the respective upper ends of the openings. The studs located below the upper end of the openings comprise the recessed areas for receiving three ball shaped members. The recessed area, the V-groove, and the sliding surface rest on the ball shaped members and ensure stress-free, positively guided relative expansion or contraction of the measurement table with respect to the mirror body. In that embodiment three half-balls mounted on the upper surface of the mirror body provide the upper support points for a receptacle or a substrate placed on them. The balls can be attached to the upper surface of the mirror body by adhesive bonding or sintering or any other suitable method.
In a particular embodiment of the measurement table it can be used universally for both reflected and transmitted light applications, such as, for example, coordinate measuring instruments and wafer steppers. Since the measurement table, the mirror body, and the receptacle each have centrally located frame-shaped openings lined up above one another, the substrate placed on the measurement table is easily accessible from above and below for reflected and transmitted light applications. The embodiment is also advantageous in that the openings provided in the mirror body decrease the weight of the mirror body and reduce the costs of manufacture.